Seaweed extract reshapes zinc oxide nanoparticles, controlled study shows

Zinc oxide nanoparticles are among the most widely used engineered nanomaterials on the planet, turning up in sunscreens, ceramics, electronics, food packaging, and experimental water-treatment systems. Yet the same material can behave in radically different ways depending on how it is made. A new open-access study published in Discover Chemistry by researchers at the Instituto Politécnico Nacional in Querétaro, Mexico, offers one of the cleanest head-to-head comparisons to date, showing that a simple extract of the nuisance seaweed Sargassum can fundamentally rewrite the size, shape, surface chemistry, and optical behavior of zinc oxide nanoparticles — even when every other synthesis variable is held constant.

The problem the team set out to solve is a stubborn one in nanoscience. Over the past decade, dozens of papers have compared biologically synthesized zinc oxide nanoparticles with chemically synthesized ones, and many have reported striking differences. But in most of those studies, the biological and chemical routes differed not just in the presence of a plant or algal extract — they also used different zinc precursors at different concentrations, different pH values, different temperatures, and different reaction times. When so many parameters change at once, it becomes impossible to say whether the observed differences come from the biological extract itself or simply from the altered reaction chemistry. The Mexican team, led by J. D. A. Loa and N. G. Rojas-Avelizapa, designed their experiment to eliminate that ambiguity.

Their approach was elegantly simple. Both batches of nanoparticles were produced by alkaline precipitation under identical conditions: the same 0.3 M zinc nitrate precursor, the same pH of 10, the same room-temperature reaction, the same two-hour reaction time, the same centrifugation and washing steps, and the same drying protocol at 60 °C with no calcination. The only difference was that in the biological synthesis, 20 milliliters of an aqueous Sargassum extract — prepared by acidifying and heating the washed, dried, pulverized seaweed biomass — replaced an equivalent volume of distilled water. Any difference between the two materials could therefore be attributed to the extract and its biomolecules, rather than to confounding synthesis parameters.

The choice of Sargassum was no accident. This brown macroalga, which has become infamous for its massive blooms washing up on Caribbean and Gulf of Mexico coastlines, is rich in sulfated polysaccharides such as fucoidan and carrageenan, along with alginates and laminarin. These macromolecules carry highly electronegative sulfate and carboxylate functional groups that can bind zinc ions, initiate electrostatic nucleation, and then wrap around growing particles as templates or capping agents. In other words, the seaweed extract is not a passive additive; it is an active participant in the architecture of the resulting nanomaterial.

The optical fingerprints of the two materials diverged immediately. Under ultraviolet-visible spectroscopy, the biologically synthesized particles, which the authors call B-ZnO-NPs, showed an absorption maximum at 354 nanometers, while the chemically synthesized C-ZnO-NPs absorbed at 369 nanometers. That fifteen-nanometer blue shift is consistent with smaller particles in the biological batch, but the researchers caution that absorption in zinc oxide is also shaped by morphology, surface chemistry, crystalline quality, and defect-related electronic states. Notably, the absorption band of the chemically synthesized particles was broader and less well defined, a pattern the authors associate with lower crystal lattice quality in the absence of organic stabilizers.

Band-gap analysis using Tauc plots reinforced the picture. The biologically derived particles exhibited an apparent optical band gap of 3.24 electron volts, compared with 3.02 electron volts for the chemically derived ones. The chemical batch actually fell below the band gap of bulk zinc oxide, about 3.37 electron volts, which the researchers interpret as evidence of structural defects in the crystal lattice — vacancies and interstitials that create recombination sites for charge carriers and shift the energy levels of the material. Because the band gap determines the photon energy needed to photoactivate the semiconductor, these differences could translate directly into different photocatalytic and antimicrobial performance, although the authors emphasize that direct functional assays remain to be done.

Fluorescence spectroscopy added another layer of evidence. Excited at 365 nanometers, both materials emitted in the yellow-orange range, with maxima at 569 nanometers for the biological particles and 574 nanometers for the chemical ones. Emission in this range is not the signature of pristine, defect-free zinc oxide, which glows near 370 nanometers from direct excitonic recombination. Instead, it points to defect-mediated transitions involving oxygen interstitials, oxygen or zinc vacancies, zinc interstitials, or surface states. The slight blue shift of the biological particles’ emission may reflect surface states generated by the organic coating inherited from the Sargassum extract, which has been reported to emit between 400 and 580 nanometers in coated zinc oxide systems.

Infrared spectroscopy confirmed the formation of the zinc-oxygen bond in both materials, with characteristic bending vibrations between 400 and 575 wavenumbers, but also revealed subtle shifts suggesting increased rigidity in some zinc-oxygen double bonds. Crucially, the biological particles showed additional features in the hydroxyl and carbonyl regions consistent with organic functional groups from the seaweed interacting with the nanoparticle surface — chemical evidence of a possible bio-derived capping layer, though the authors note that techniques such as X-ray photoelectron spectroscopy and thermogravimetric analysis would be needed to confirm and quantify it.

The colloidal and morphological measurements were perhaps the most dramatic. Dynamic light scattering showed that the biological particles carried a zeta potential of −28.65 millivolts, well beyond the −14.07 millivolts of the chemical batch, indicating much stronger electrostatic repulsion between particles and therefore better colloidal stability. The polydispersity index told a similar story: 0.59 for the biological material versus 0.96 for the chemical one, the latter signaling high heterogeneity. Transmission electron microscopy then revealed why. The biologically synthesized particles were small, granular, and relatively uniform, averaging about 34 nanometers in diameter, while the chemically synthesized particles grew into larger cubic and rod-shaped structures averaging roughly 158 nanometers in length — a sign of uncontrolled nucleation and growth in the absence of stabilizing agents.

The authors are careful about the limits of their conclusions. The contribution of the Sargassum extract should be read as an integral effect, they write, because local reaction chemistry — ionic strength, viscosity, conductivity, buffering — was not measured, and the extract may influence the reaction environment in ways beyond simple surface capping. Complementary analyses, including X-ray diffraction, high-resolution electron microscopy with elemental mapping, and spectroscopic surface probes, would be needed to pin down crystallinity, defect identity, and the precise chemical nature of the organic coating. Functional assays testing photocatalytic dye degradation, antimicrobial activity, and wastewater treatment performance would then be required to connect the observed physicochemical differences to real-world applications.

Even with those caveats, the study delivers a message with broad implications. It demonstrates, under rigorously controlled conditions, that the biochemical composition of a biological precursor is not a footnote in nanoparticle synthesis — it is a design lever. Sulfated polysaccharides and other acidic biomolecules from Sargassum appear to control nucleation, modulate growth, and stabilize the surface, yielding smaller, more homogeneous, more colloidally robust particles with distinct optical properties. For a field often criticized for irreproducible green-synthesis claims, the work offers a template: hold every parameter constant, change only the biological ingredient, and measure everything. It also hints at a satisfying circularity — turning an invasive seaweed that clogs beaches into a precision tool for engineering nanomaterials whose properties can be tuned for environmental remediation, sensing, and medicine. As the authors conclude, selecting the right biological precursor and synthesis strategy may be the key to obtaining zinc oxide nanoparticles whose characteristics are tailored, rather than left to chance.

Subject of Research: Physicochemical comparison of zinc oxide nanoparticles synthesized biologically with Sargassum extract versus chemically under identical conditions

Article Title: Physicochemical comparison of zinc oxide nanoparticles obtained by biological and chemical synthesis

Article References: Loa, J. D. A., Guatemala-Cisneros, M. E., Hernández-Jiménez, M. O., & Rojas-Avelizapa, N. G. (2026). Physicochemical comparison of zinc oxide nanoparticles obtained by biological and chemical synthesis. Discover Chemistry, 3(1), Article 470. https://doi.org/10.1007/s44371-026-00922-z

Image Credits: AI Generated

DOI: 10.1007/s44371-026-00922-z

Keywords: zinc oxide nanoparticles, Sargassum, green synthesis, chemical precipitation, band gap, zeta potential, colloidal stability, transmission electron microscopy, photocatalysis, nanobiotechnology, sulfated polysaccharides, defect chemistry